Adaptive precoder cycling

ABSTRACT

Methods and apparatus are disclosed for improving a precoder selection process in a wireless communications system. In a normal precoder selection process, a precoder is selected from a codebook based on channel state information estimated from received reference signals. In between two received reference signals, a subset of precoders is cycled through and each precoder in the subset is iteratively selected for use by a transmitter to precode transmit signals. The subset of precoders may be adaptively modified based on predefined criteria.

TECHNICAL FIELD

The present invention relates generally to precoding transmit signalsand, more specifically, to selection of a precoder from a set ofpre-defined matrices to precode transmit signals in a multiple-inputmultiple-output (MIMO) wireless communications system.

BACKGROUND

A MIMO communications system employs multiple antennas at both thetransmitter and the receiver. A MIMO system can be used to achieveimproved system capacity, signal coverage and higher user data rates. AMIMO system also provides the possibility of very high bandwidthutilization.

A MIMO system comprises NT transmit antennas and NR receive antennas.Multiple signal streams are transmitted from the transmit antennas andreceived by the receive antennas. Each signal stream in the MIMO systemis transmitted in a separate layer. In an NT×NR MIMO system, the numberof signal streams or layers, NL, is smaller than or equal to min {NT,NR}. Generally, an MIMO transmitter utilizes precoding techniques toprecode transmit signals with a precoding matrix. A precoding matrix isalso referred to as a precoder in the present application. Both termsare used interchangeably. Precoding techniques help reduce interferencebetween the layers and improve signal-to-noise ratio at the receiver.

In practice, a set of precoders may be compiled to form a predefinedcodebook. Based on channel state information (CSI) estimated by areceiver, a suitable precoder can be selected from the codebook toprecode transmit signals at a transmitter.

Often, channel state information is derived from received referencesignals. For example, demodulation reference signals (DM-RS) or soundingreference signals (SRS) can be used in channel state informationestimation. However, DM-RS signals are precoded. The channel stateinformation obtained from a precoded DM-RS reference signal maymisrepresent the actual channel conditions of the physical channels.Further, the reference signals may be transmitted too infrequently toallow prompt or timely estimation of channel state information.

When a precoder is selected based on incomplete, inaccurate, or stalechannel state information, the precoder may be less effective incancelling interference between signal streams or in improving thesignal-to-interference-plus-noise-ratio at the receiver. The transmitsignals may exhibit high interference or high noise level.

Accordingly, techniques are needed to improve precoder selections in aMIMO system when channel state information estimated from certain typesof reference signals does not accurately reflect the current channelconditions.

SUMMARY

The present disclosure provides methods and apparatus for improvingprecoder selection in a wireless communications system. Morespecifically, particular embodiments of the present invention provide aprecoder selection method that periodically selects one or moreprecoders from a subset of precoders in addition to a normal precoderselection process. The method may be implemented in a transmitter or areceiver.

In some implementations, the normal precoder selection process selects aprecoder from a pre-defined codebook based on channel state informationestimated from certain received reference signals. A subset of precodersmay be constructed from a pre-defined codebook. The subset of precodersis cycled through during each duty period when each precoder isiteratively selected from the subset. The precoder cycling is performedevery cycling period. The selection of a precoder by cycling through thesubset of precoders may be performed by a receiver or a transmitter. Theselected precoder is used by the transmitter to precode transmitsignals.

In some implementations, the subset of precoders is adaptively modifiedby adding or deleting one or more precoders. The addition or deletionmay be based on a pre-defined criterion.

Of course, the present invention is not limited to the features,advantages, and contexts summarized above, and those familiar withwireless communications systems and technologies will recognizeadditional features and advantages upon reading the following detaileddescription and upon viewing the accompanying drawings. Notably, whileterminologies specific to 3GPP LTE are used in this application, theideas and concept of the invention are equally applicable to otherwireless systems, including but not limited to WCDMA, WiMax, UMA, GSMand WLAN.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a block diagram of a MIMO communications system.

FIG. 2 illustrates an exemplary network node.

FIG. 3 illustrates an exemplary user terminal.

FIGS. 4 and 5 illustrate exemplary codebooks for spatial multiplexing.

FIG. 6 illustrates an exemplary method of precoder cycling.

FIG. 7 illustrates another exemplary method of precoder cycling.

FIG. 8 illustrates two exemplary precoder cycling schedules.

FIG. 9 illustrates an exemplary method of adaptive precoder cycling.

FIG. 10 illustrates an exemplary throughput performance for non-adaptiveprecoder-cycling.

FIG. 11 illustrates an exemplary throughput performance for adaptiveprecoder cycling.

FIG. 12 illustrates an exemplary adaptive precoder cycling process.

DETAILED DESCRIPTION

Referring now to the drawings, FIG. 1 illustrates a MIMO communicationssystem. A transmitter 102 is equipped with multiple antennas 104. Themulti-antenna transmitter 102 utilizes a precoding matrix, V, to precodethe transmit signals {right arrow over (S)}_(N) _(L) before transmittingthe signals via the antennas 104. The precoding matrix, V, is applied toNL input signals to generate NT transmit signals.

The precoded signal is a multi-stream signal and comprises NL layers.The precoded signal propagates through the physical channels. Thechannel conditions of the channels are represented by channel responseH. The precoded signal is received by a receiver 106. The receiver 106is also equipped with multiple antennas 108. At the receiver 106, thereceived signals are demodulated and combined with combining weights W.The relationship between the transmit signal {right arrow over (S)}_(N)_(L) and the demodulated signal {right arrow over (r)}_(N) _(L) is shownin Equation (1).{right arrow over (r)}_(N) _(L) =W·H·V·{right arrow over (S)} _(N) _(L)  (1)

Theoretically, with a carefully chosen precoder, V, impairments due tointerference between the multiple signal streams can be effectivelyeliminated or reduced in the signals received in the receiver 106. Forinstance, precoder V can be selected such that W·H·V becomes a diagonalmatrix, in which case each signal stream in the received signal {rightarrow over (r)}_(N) _(L) is linearly related to the corresponding signalstream in the transmit signal {right arrow over (S)}_(N) _(L) withoutinterference from other multiple signal streams.

In a MIMO system, a multi-antenna receiver estimates channel stateinformation (CSI) based on reference signals transmitted by amulti-antenna transmitter. Examples of reference signals include SRS andDM-RS signals. Based on the estimated CSI, the receiver selects aprecoder and informs the transmitter of the selected precoder for use inprecoding transmit signals. Alternatively, the receiver can transmit theCSI estimates to the transmitter. The transmitter selects a precoderbased on the CSI estimates.

FIG. 2 illustrates an exemplary receiving device 200 in a MIMO systemimplementing adaptive precoder cycling as hereinafter described. Foruplink communications, the receiving device may comprise a base station(also known as a NodeB). For downlink communications, the receivingdevice may be in a wireless terminal (also known as a user equipment).

The receiving device comprises a transceiver circuit 218, a signalprocessing circuit 212, and a plurality of antennas. The transceivercircuit includes a receiver to receive signals from a transmittingdevice on one or more antennas, and a transmitter to transmit signals tothe transmitting device using one or more antennas. The signalprocessing circuit 212 processes the signals received and transmitted bythe receiving device. The signal processing circuit 212 may comprise oneor more processors, microcontroller, hardware, or a combination thereof.The signal processing circuit 212 comprises a CSI estimator 214 and anoptional precoder selector 216 for implementing codebook-basedprecoding. As will be hereinafter described in more detail, the CSIestimator 214 generates estimates of channel state information based ona received reference signals. The precoder selector 216 then selects aprecoder from a pre-compiled codebook based on the channel stateinformation estimated by the CSI estimator 214. The optional precoderselector 216 may be included in the transmitting device, in which casethe CSI is signaled to the transmitting device.

FIG. 3 illustrates an exemplary transmitting device 300 in a MIMO systemimplementing adaptive precoder cycling as hereinafter described. Foruplink communications, the transmitting device may comprise a userterminal or user equipment. For downlink communications, thetransmitting device may comprise a base station or NodeB.

The transmitting device 300 comprises a transceiver circuit 326, asignal processing circuit 322, and a plurality of antennas. Thetransceiver circuit 326 includes a receiver to receive signals on one ormore antennas, and a transmitter to transmit signals using one or moreantennas. The signal processing circuit 322 processes the signalsreceived and transmitted by the receiving device. The signal processingcircuit may comprise one or more processors, microcontroller, hardware,or a combination thereof. The signal processing circuit includes aprecoding circuit 324 to precode the transmit signals. Optionally, thetransmitting device 300 may further comprise a precoder selector 316similar to the precoder selector 216 included in the receiving device200. The precoder selector 316 selects a precoder based on the CSI thatis estimated by the CSI estimator 214 and transmitted from the receivingdevice 200 to the transmitting device 300.

As note above, the receiving device 200 and transmitting device 300 areconfigured to implement codebook-based precoding. With codebook basedprecoding, a set of predefined precoders are stored in a codebook. Thereceiving device selects a precoder from the codebook based on thecurrent channel conditions and sends the precoder selection to thetransmitting device. The transmitting device may either accept theprecoder selection received from the receiving device, or select adifferent precoder.

FIGS. 4 and 5 illustrate exemplary codebooks that may be employed in aMIMO system. The exemplary codebooks as shown in FIGS. 4 and 5 are atable containing two indices, PMI (Precoding Matrix Indicator) and RI(Rank Indicator). The values in each column, r, represent antennaspreading weights, i.e., the precoder coefficients for the r-th layer.The PMI index is an index for the antennas. For example, in FIG. 4, incolumn r=1, all values are empty except element (0, 1), which has avalue of

${\frac{1}{\sqrt{2}}\begin{bmatrix}1 & 0 \\0 & 1\end{bmatrix}}.$This indicates that there is only one antenna transmitting (PMI=0). FIG.4 illustrates an exemplary codebook including precoder matrices that canbe used for a MIMO system capable of transmitting up to 2 layers. FIG.5, on the other hand, illustrates an exemplary codebook that can be usedfor a MIMO system capable of transmitting up to 4 layers.

Based on the estimated channel state information, the transmittingdevice 200 or the receiving device 300 selects a precoding matrix from acodebook, such as the exemplary codebook shown in FIG. 4 or 5. Theprecoder selection takes place upon receiving a reference signal thatcan be used for CSI estimation, for example, a DM-RS reference signal ora SRS reference signal.

DM-RS reference signals are transmitted on a Physical Uplink SharedChannel (PUSCH) and are precoded. The CSI obtained from DM-RS referencesignals does not reflect the channel conditions of the physicalchannels. On the other hand, SRS reference signals are not precoded andthe CSI information obtained from SRS reference signals providesknowledge of the physical channels. However, SRS reference signalsrequire extra signaling overhead and are not transmitted as frequentlyas desired to ensure accurate or timely CSI estimation. The desiredfrequency of SRS transmissions depends on how fast the channelconditions and/or interference vary. The Doppler spread of a channeloften is a good indicator of the scale of time variation of the channelconditions and interference.

In some embodiments, the frequency of SRS transmissions may beconfigured to be once every 2 ms to once every 320 ms. When SRStransmissions are too infrequent to guarantee accurate CSI estimation,DM-RS reference signals can be used to enhance CSI estimations. Thedrawback of using DM-RS for CSI estimation is that the CSI obtained fromDM-RS reference signals is mainly confined to the precoder used by thePUSCH subframe. Therefore, if the PUSCH uses a certain precoding matrixselected based on the CSI information obtained from the DM-RS signals,the same precoding matrix may be selected if the PUSCH channel conditionremains relative stable. And if the same precoding matrix has been usedfor a while, there is no opportunity to measure or update the CSIinformation corresponding to other precoder matrices. Therefore, onlywhen newly measured channel state information, e.g., SINRs for the PUSCHprecoder, becomes smaller than the SINRs maintained for the otherprecoders, which have not been updated for a while since these precodershave not been used, will a different precoding matrix be selected. Theprecoder selected based on outdated SINR information may be less thanoptimal in reducing interference.

To overcome such drawbacks, a subset of precoding matrices may beselected and the transmitter may be required to cycle through theprecoder subset according to a cycling pattern. The cycling pattern maybe described by a cycling period, a duty period, and a cycling offset.The cycling period describes how often cycling through of the subset ofprecoding matrices takes place. The duty period describes the period oftime it takes to cycle through the subset of precoding matrices. And thecycling offset describes the separation time between the last SRStransmission and the start of a precoder cycling period. For example,FIG. 6 illustrates an exemplary cycling pattern over a series ofsubframes. The precoding matrix selected for each subframe is indicatedby the RI and PMI index of the precoding matrix. The RI index is shownin the upper line of boxes and the PMI index is shown in the lower lineof boxes. The shaded subframes are subframes during which precodercycling is activated and the un-shaded subframes are subframes whennormal precoder selection is used.

In FIG. 6, the subset of precoders is selected from the codebook shownin FIG. 4. The subset of precoders includes{(RI,PMI)}={(0,0),(0,1),(0,2),(0,3),(1,0)}  Eq. (2).In this example, precoders (RI, PMI)=(0, 4), (0, 5) are not included inthe subset because they yield small or unacceptable SINRs. As shown inFIG. 6, the first cycling period 620 starts with precoder (RI, PMI)=(0,3) in subframe i, the first subframe in duty period 610. After precoder(RI, PMI)=(0, 3), precoder (RI, PMI)=(1, 0) is selected for use in thenext subframe, subframe i+1. Similarly, the second cycling period startswith using precoder (RI, PMI)=(0, 0) in the first subframe, subframe j,in the cycling period after the cycling period 620. This is because (RI,PMI)=(0, 0) is the precoder after (RI, PMI)=(1, 0) in the precodersubset shown in Eq(2). Precoder (RI, PMI)=(1, 0) is used in the subframeright before subframe j.

In the embodiment shown in FIG. 7, only the RI index is cycled. The PMIindex remains the same. This tends to improve the performance of theMIMO system, in cases where RI selection plays a more important rolethan PMI selection in affecting the system performance or where PMIcycling consumes too much overhead.

In some embodiments, the cycling pattern can be varied depending on thecurrent conditions. Of course, the transmitting device 300 and thereceiving device 200 need to be informed of the varied cycling patternin advance. In general, the longer the cycling pattern, the more CSIinformation the receiving device 200 can gather. But the longer thecycling pattern, the more signaling overhead the MIMO system has toprovide. For example, in FIG. 6 and FIG. 7, the cycling duty is 4subframes and 1 subframe respectively. The ratio of the duty period tothe cycling period determines the cycling overhead. Higher cycling dutyperiod or lower cycling period means higher cycling overhead. In otherwords, it is desirable to make the cycling duty period as short aspossible, while cycling all necessary precoding matrices within the dutyperiod.

In some implementations, the cycling offset is configured such that theprecoder cycling can complement infrequent SRS transmission. Forexample, as illustrated in FIG. 8 a, precoder cycling may be scheduledsomewhere midway in between two consecutive SRS transmissions, insteadof immediately before or after an SRS transmission, as shown in FIG. 8b. The receiver performance comparison between these two options maydepend on many systems variables such as Doppler shift and receivedSINR.

It is advantageous to adapt the precoder cycling based on channelconditions and system performance. Adaptive precoder cycling avoidsunnecessary precoder cycling if a selected precoding matrix is adequateor satisfactory. Precoder cycling parameters, such as cycling period,duty period and cycling offset, may be adapted dynamically. The subsetof precoder matrices can be modified dynamically. For example, if aprecoding matrix used in a precoder cycling is seldom selected by thenormal precoder selection, it may be excluded from the subset used inprecoder cycling. The assumption is that such precoding matrix isprobably not a good match for the prevailing channel conditions. Byremoving the unsuitable precoding matrices from precoder cycling, theduty period can be shortened and the cycling overhead can be reduced.

FIG. 9 illustrates one exemplary adaptive precoder cycling. In the firstprecoder cycling period, the adaptive precoder cycling comprises oneprecoding matrix (RI, PMI)=(1, 0). However, the normal precoderselection uses only (RI, PMI)=(0, 1) or (0, 2). The precoder (RI,PMI)=(1, 0) is never selected. It may be assumed that the precodingmatrix (RI, PMI)=(1, 0) does not optimize the system performance andshould not be used in the precoder cycling. As shown in FIG. 9, in thesecond precoder cycling period, the precoding cycling is skippedentirely. In some implementations, each precoding matrix in the subsetis tracked to see how many times it is used over a cycling period. Autilization metric may be maintained for each precoder in the subset.The utilization metric describes the extent to which each pre-coder hasactually been selected for use by the transmitter. If one precodingmatrix is never used during a previous cycling period, that precodingmatrix may be excluded from the subset. This helps shortening the dutyperiod, reducing the signaling overhead associated with precodercycling, and improves system throughput, as illustrated in FIGS. 8 and9.

FIG. 10 shows the simulated throughput performance as a function of thereceived SNR in a non-adaptive precoder selection scheme. The precoderselection is based on both DM-RS and SRS signals. The dotted linesrepresent the throughput performance when there is no precoder cyclingand the solid lines represent the throughput performance whennon-adaptive precoder cycling is used. In FIG. 10, the curves withcircle marks represent the performance with a SRS periodicity of 320 msand the curves with square marks represent the performance with a SRSperiodicity of 20 ms. As shown in FIG. 10, without precoder cycling, thethroughput performance increases as the SRS periodicity increases. Withprecoder cycling, the receiver becomes less sensitive to SRSperiodicity. This may be due to the improved accuracy in CSI estimationwhen periodically different precoding matrices are selected for CSIestimation.

It should be noted that precoder cycling may cause performance loss in ascenario in which the SNR is high. This is because the normal precoderselection almost always selects the full-rank precoding matrix andprecoder cycling may select low-rank precoding matrices that are notsuitable for the high SNR scenario. In such case, non-adaptive precodercycling may actually lead to throughput loss. Adaptive precoder cyclingshould be implemented instead.

FIG. 11 illustrates the simulated throughput performance comparisonbetween an adaptive precoder selection scheme and a non-adaptiveprecoder selection scheme. In FIG. 11, the dotted lines representnon-adaptive precoder selection and the solid lines represent adaptiveprecoder selection. Same as in FIG. 10, the curves with circle marksrepresent the performance with a SRS periodicity of 320 ms and thecurves with square marks represent the performance with a SRSperiodicity of 20 ms. In FIG. 11, in the high SINR region, thethroughput performance of adaptive precoder cycling is higher than theperformance of non-adaptive precoder cycling.

FIG. 12 illustrates an exemplary adaptive precoder cycling processaccording to one embodiment of the invention. The selection process canbe implemented by the precoder selector 216 or 316. A subset ofprecoders is first constructed for use during precoder cycling (step1202). During a cycling period, each precoder in the subset of precodersis iteratively selected to precode transmit signals at a transmitter(step 1204). The subset of precoders is adaptively or dynamicallymodified before the subset of precoders is cycled through again (step1206).

The foregoing description and the accompanying drawings representnon-limiting examples of the methods and apparatus taught herein. Assuch, the present invention is not limited by the foregoing descriptionand accompanying drawings. Instead, the present invention is limitedonly by the following claims and their legal equivalents.

What is claimed is:
 1. A method for selecting a pre-coder to be used bya transmitter to pre-code transmit signals in a multiple-input multipleoutput (MIMO) communications system, comprising: constructing a subsetof pre-coders from a predetermined codebook that includes a plurality ofpre-coders; cycling through the subset of pre-coders during a cyclingperiod by iteratively selecting individual pre-coders in the subset ofpre-coders to be used by the transmitter for pre-coding a referencesignal, wherein for each iteratively selected precoder, channel stateinformation is updated based on measurement of the reference signal andperiodic measurements of a second reference signal that has not beenpre-coded by the transmitter, wherein said cycling through the subset ofpre-coders is performed in between consecutive ones of the periodicmeasurements, and wherein each iteratively selected pre-coder is fedback from the receiver to the transmitter; and adaptively modifying thesubset of pre-coders to be cycled through, by adding to the subset oneor more suitable pre-coders or deleting from the subset one or moreunsuitable pre-coders, based on a predetermined criterion.
 2. The methodof claim 1, wherein the method is implemented in the transmitter, andwherein for each iteratively selected precoder, channel stateinformation is updated based on a measurement of the reference signaland fed back from a receiver to the transmitter.
 3. The method of claim1, wherein the predetermined criterion comprises a utilization metricfor each of the subset of pre-coders describing the extent to which thatpre-coder has actually been selected for use by the transmitter based onsaid channel state information.
 4. The method of claim 3, wherein saidadaptively modifying comprises removing one or more pre-coders from thesubset having a utilization metric indicating that the pre-coder has notbeen selected for at least a threshold number of times over apredetermined interval of time.
 5. The method of claim 4, wherein saidcycling is performed during each of a plurality of cycling periods, andwherein said predetermined interval of time is a predetermined number ofcycling periods.
 6. The method of claim 1, wherein said cycling isselectively performed if at least two pre-coders are included in saidsubset.
 7. The method of claim 1, wherein the constructed subset ofpre-coders includes every pre-coder defined in the predeterminedcodebook.
 8. A wireless multi-antenna transceiver configured to select apre-coder for pre-coding transmit signals, said transceiver comprising atransmitting and receiving circuit, a signal processing circuit, saidsignal processing circuit configured to: construct a subset ofpre-coders from a predetermined codebook that includes a plurality ofpre-coders; cycle through the subset of pre-coders by iterativelyselecting individual pre-coders in the subset of pre-coders forpre-coding a reference signal, wherein for each iteratively selectedprecoder, channel state information is updated based on measurement ofthe reference signal and periodic measurements of a second referencesignal that has not been pre-coded, wherein said cycling through thesubset of pre-coders is performed in between consecutive ones of theperiodic measurements, and wherein each iteratively selected pre-coderis fed back from the transceiver to a transmitter to pre-code transmitsignals; and adaptively modify the subset of pre-coders to be cycledthrough, by adding to the subset one or more suitable pre-coders ordeleting from the subset one or more unsuitable pre-coders, based on apredetermined criterion.
 9. The transceiver of claim 8, the signalprocessing circuit further configured to receive from a receiver updatedchannel state information based on a measurement of the referencesignal.
 10. The transceiver of claim 8, wherein the predeterminedcriterion comprises a utilization metric for each of the subset ofpre-coders describing the extent to which that pre-coder has actuallybeen selected for use by the transmitter based on said channel stateinformation.
 11. The transceiver of claim 10, wherein said adaptivelymodifying comprises removing one or more pre-coders from the subsethaving a utilization metric indicating that the pre-coder has not beenselected for at least a threshold number of times over a predeterminedinterval of time.
 12. The transceiver of claim 11, wherein said cyclingis performed for each of a plurality of cycling periods, wherein saidpredetermined interval of time is a predetermined number of cyclingperiods.
 13. The transceiver of claim 11, wherein said cycling isselectively performed if at least two pre-coders are included in saidsubset.
 14. The transceiver of claim 11, wherein the constructed subsetof pre-coders includes every pre-coder defined in the predeterminedcodebook.